[llvm] [MachineLoopInfo] Fix getLoopID to handle multi latches. (PR #106195)

[Marc Auberer via llvm-commits]

================
@@ -211,8 +211,6 @@ MDNode *MachineLoop::getLoopID() const {
             break;
           }
         }
----------------
marcauberer wrote:

I think this won't work for a case like this:

```
bb1:                    ; head
  br %bb3

bb2:                    ; latch
  br %bb1 !loop !1

bb3:
  br %bb2
```

As far as I understand, the BasicBlocks are visited in no particular order that ensures, that latch blocks are visited last.
This means that if we visit `bb2`, we set `LoopID` to a non-null value. After that, we visit `bb3`, which has no metadata attached / is no latch => MD is nullptr. Thus the check in line 218 succeeds and we return from the function, which is not what we want.

I think this should work better:
```suggestion
        }
        if (!MD)
          continue;
```

Maybe you can also add a test case for this scenario?

https://github.com/llvm/llvm-project/pull/106195